Controlling the temperature of a blackbody radiator allows the generation of a determinable amount of infrared energy at known wavelengths. Certain species of gases and liquids absorb a determined amount of infrared energy at unique wavelengths as a function of path length and concentration. Detection of this absorbance has been successfully used to determine concentration values of the species of interest. Infrared analyzers precisely and accurately control the source temperature with current and voltage regulation devices. Great caution in design is exercised to assure the source temperature is constant in all modes of operation and calibration. The prior art devices require interrupting on-line process measurements to perform time-consuming and costly calibration and operational checks, to the detriment of production efficiency. In critical operations, the use of redundant infrared analyzers minimize process down-time but at the expense of added cost and complexity. A more reliable method and apparatus that does not require lengthy and costly interruptions for calibration and operational checks are needed.
Infrared analysis of a wide range of gases and liquids is a well-known art, relying on the physical phenomenon that many species absorb infrared energy at specific wavelengths. For example, carbon dioxide (CO.sub.2) absorbs infrared energy at about 4.25 microns; carbon monoxide (CO) absorbs infrared energy at about 4.63 microns. The amount of infrared energy absorbed by the species is a direct function of the gas or liquid concentration and path length of the species at its unique wavelength.
In the past, there have been only two reported methods of checking the calibration of infrared analyzers. The first, and most common method, is to introduce a known gas or liquid standard solution into the measuring cell of the infrared analyzer. The decrease in infrared energy measured at the infrared detector at the specific wavelength in question allows the instrument to be properly spanned related to concentration values. In order to check the calibration of an operating infrared analyzer, general practice is to by-pass the stream sampled and divert standard solutions of gases or liquids to the infrared analyzer measuring cells through suitable valves, either manually or electrically controlled, and ascertain that correct calibration values are achieved.
The second method is to insert a calibrating filter into the measuring path between the infrared light source and the infrared detector. The calibrating filter is manufactured to produce a defined absorbance of the infrared energy at the wavelength of interest, thus affording the condition to properly span the infrared analyzer to the simulated gas or liquid concentration desired or to check the calibration or operation of the analyzer by momentarily inserting the filter.
The first method is cumbersome, expensive and the handling of gas cylinders is usually hazardous.
The second method requires solenoids, motors or other mechanical devices which are cumbersome, expensive, use significant amounts of power and after long periods of un-attendance and/or the neglect of lubrication, they become gummy, sticky and generally inoperative.
The problems of cost and complexity enumerated in the foregoing with respect to the above prior art systems are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known devices and methods for checking the calibration of infrared analyzers operating on liquid or gas process streams. Other noteworthy problems may exist. However, those presented above should be sufficient to demonstrate that devices and methods for checking the calibration of infrared analyzers appearing in the prior art have not been altogether satisfactory.
It is an object of this invention to provide an improved method and apparatus for checking the calibration of analyzers herein described.
It is a particular object of this invention to provide an improved method and apparatus for checking the calibration of infrared analyzers for determining specific constituent content of gases and liquids.
Other objects will become apparent from the description of the invention as set forth herein.